The present invention relates to an internal combustion engine.
In the past, in an internal combustion engine, for example, a diesel engine, the production of NOx has been suppressed by connecting the engine exhaust passage and the engine intake passage by an exhaust gas recirculation (EGR) passage so as to cause the exhaust gas, that is, the EGR gas, to recirculate in the engine intake passage through the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount of heat, so the larger the amount of EGR gas, that is, the higher the EGR rate (amount of EGR gas/(amount of EGR gas + amount of intake air), the lower the combustion temperature in the combustion chamber. When the combustion temperature falls, the amount of NOx produced falls and therefore the higher the EGR rate, the lower the amount of NOx produced.
In this way, in the past, it was known that the higher the EGR rate, the lower the amount of NOx produced can become. If the EGR rate is increased, however, the amount of soot produced, that is, the smoke, starts to sharply rise when the EGR rate passes a certain limit. In this point, in the past, it was believed that if the EGR rate was increased, the smoke would increase without limit. Therefore, it was believed that the EGR rate at which smoke starts to rise sharply was the maximum allowable limit of the EGR rate.
Therefore, in the past, the EGR rate was set within a range not exceeding the maximum allowable limit. The maximum allowable limit of the EGR rate differed considerably according to the type of the engine and the fuel, but was from 30 percent to 50 percent or so. Accordingly, in conventional diesel engines, the EGR rate was suppressed to 30 percent to 50 percent at a maximum.
Since it was believed in the past that there was a maximum allowable limit to the EGR rate, in the past the EGR rate had been set within a range not exceeding that maximum allowable limit so that the amount of NOx and smoke produced would become as small as possible. Even if the EGR rate is set in this way so that the amount of NOx and smoke produced becomes as small as possible, however, there are limits to the reduction of the amount of production of NOx and smoke. In practice, therefore, a considerable amount of NOx and smoke continues being produced.
In the process of studying the combustion in diesel engines, however, it was discovered that if the EGR rate is made larger than the maximum allowable limit, the smoke sharply increases as explained above, but there is a peak to the amount of the smoke produced and once this peak is passed, if the EGR rate is made further larger, the smoke starts to sharply decrease and that if the EGA rate is made at least 70 percent during engine idling or if the EGR gas is force cooled and the EGR rate is made at least 55 percent or so, the smoke will almost completely disappear, that is, almost no soot will be produced. Further, it was found that the amount of NOx produced at this time was extremely small. Further studies were engaged in later based on this discovery to determine the reasons why soot was not produced and as a result a new system of combustion able to simultaneously reduce the soot and NOx more than ever before was constructed. This new system of combustion will be explained in detail later, but briefly it is based on the idea of stopping the growth of hydrocarbons into soot at an intermediate stage before the hydrocarbons grow into soot.
That is, what was found from repeated experiments and research was that the growth of hydrocarbons stops at an intermediate stage before becoming soot when the temperature of the fuel and the surrounding gas at the time of combustion in the combustion chamber is lower than a certain temperature and the hydrocarbons grow to soot all at once when the temperature of the fuel and the surrounding gas becomes higher than a certain temperature. In this case, the temperature of the fuel and the surrounding gas is greatly affected by the heat absorbing action of the gas around the fuel at the time of combustion of the fuel. By adjusting the amount of heat absorbed by the gas around the fuel in accordance with the amount of heat generated at the time of combustion of the fuel, it is possible to control the temperature of the fuel and the surrounding gas.
Therefore, if the temperature of the fuel and the surrounding gas at the time of combustion in the combustion chamber is suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway, soot is no longer produced. The temperature of the fuel and the surrounding gas at the time of combustion in the combustion chamber can be suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, the hydrocarbons stopped in growth midway before becoming soot can be easily removed by after-treatment using an oxidation catalyst etc. This is the basic thinking behind this new system of combustion. A patent application for an internal combustion engine using this new combustion system has already been filed by the applicant (Japanese Patent Application No. 9-305850).
In this new system of combustion, however, the EGR rate must be made at least about 55 percent. The EGR rate can be made at least about 55 percent when the amount of suction air is relatively small. That is, this new combustion is not possible when the amount of suction air exceeds a certain level. Therefore, when the amount of suction air exceeds a certain level, it is necessary to switch to the conventionally performed combustion. In this case, if the EGR rate is lowered to switch to the conventionally performed combustion, the EGR rate will pass the range of the EGR rate where the amount of smoke produced peaks, so a large amount of smoke will be produced.
If the injection timing is delayed until after top dead center of the compression stroke under this new combustion system, however, since the temperature in the combustion chamber falls around when the injection is performed, the temperature of the fuel and its surrounding gas will not rise that much at the time of combustion. Therefore, at this time, it is found that the peak value of the amount of smoke produced becomes smaller. Therefore, it the injection timing is delayed until after top dead center of the compression stroke when switching from this new combustion to the conventionally performed combustion, it becomes possible to suppress the amount of smoke produced at the time of switching.
An object of the present invention is to provide an internal combustion engine capable of suppressing the generation of smoke when switching between new combustion and the conventionally performed combustion.
According to the present invention, there is provided an internal combustion engine in which an amount of production of soot gradually increases and then peaks when an amount of inert gas in a combustion chamber increases and in which a further increase of the amount of inert gas in the combustion chamber results in a temperature of fuel and surrounding gas in the combustion chamber becoming lower than a temperature of production of soot and therefore almost no production of soot any longer, the engine comprising switching means for selectively switching between a first combustion where the amount of inert gas in the combustion chamber is higher than the amount of inert gas where the amount of production of soot peaks and where almost no soot is produced and a second combustion where the amount of inert gas in the combustion chamber is smaller than the amount of inert gas where the amount of production of soot peaks, an injection timing being delayed to after top dead center of the combustion stroke when switching between the first combustion and second combustion.